Certain weeds such as Ipomoea, Cyperus, Sida and Euphorbia are particularly difficult to control. Their full-season competition can reduce crop yields and cause significant economic loss in row-crop production. One of the most common practices for controlling these troublesome weeds is the postemergent application of a herbicide. However, there is no single selective herbicide currently available which will give economic control of these weeds.
Glyphosate [(N-phosphonomethyl)glycine] is a highly effective herbicide with a broad spectrum of activity. However, certain agronomically important weed species require relatively high application rates of glyphosate for effective control. High rates of glyphosate can lead to undesirable increased selection pressure. Glyphosate tolerance, particularly in Ipomoea, is a recognized problem in the field. Therefore, new weed control methods and compositions which effectively lower glyphosate application rates while providing broad spectrum weed control are highly desirable.
Aqueous compositions containing imidazolinyl acid salts are described in U.S. Pat. No. 4,816,060, and aqueous compositions containing glyphosate salts are described in EP 220,902-A2. However, concentrated, aqueous compositions comprising an imidazolinyl acid salt and a glyphosate salt have not been described. The principal reason that aqueous compositions containing both of those compounds have not been disclosed is that imidazolinyl acid salts and glyphosate salts are, in general, not mutually compatible. Aqueous glyphosate salt compositions generally have a pH value of about 4. However, imidazolinyl acid salts are not entirely stable in an environment having a pH value of about 4. Conversely, glyphosate salts are not entirely stable at the pH values required to provide stable aqueous compositions of imidazolinyl acid salts.
To overcome the incompatibility problems associated with aqueous compositions containing imidazolinyl acid salts and glyphosate salts, emulsifiable suspension concentrate compositions containing imidazolinyl acids and glyphosate have been described (see, e.g., U.S. Pat. No. 5,268,352). However, emulsifiable suspension concentrate compositions are not entirety satisfactory because they require the use of heavy aromatic solvents.
What is needed in the art is an aqueous composition which overcomes the incompatibility problems associated with imidazolinyl acid salts and glyphosate salts without requiring the use of heavy aromatic solvents.